There is no such thing as locuming in the US. You have to take the USMLE. Also Uk training doesn't mean much there so you also need to enter a residency,ie you can't go from being a doc in the Uk to the equivalent there,you need US training. For level of difficulty, go to the USMLE website and look at the syllabus. It's harder than MBBS exams.
Doing a fellowship has slightly different rules, there are certain circumstances where you can do a fellowship there without having done US residency but I don't really know the details.